1. Field of the Invention
The present invention relates generally to light sources, and more particularly to an illumination module based on optical power combining to provide pump radiation to an optical frequency converter.
2. Related Art
Many optical processes of used in various illuminator applications generally show improved performance as the input optical power increases. For example, the efficiency of second harmonic generation (SHG) increases as the input power increases. Thus, providing high optical power may be desirable. High optical power may be provided by a single high power source, or by effectively combining the outputs of two or more low power sources to provide a high power combined output. This second approach is generally referred to as optical power combining.
In many cases, it is desirable for the combined output radiation to be in a single spatial mode. However, imposing the requirement of a single spatial mode on a combined output has significant consequences for optical power combining. In particular, interference between the optical inputs may occur in the combined single mode output unless the optical inputs are distinguishable (e.g., have different wavelengths and/or different states of polarization). In order to obtain power combining of interfering optical inputs, phase coherence of these optical inputs must be established, which typically requires implementation of an elaborate optical phase locking scheme. Accordingly, in applications that permit the use of distinguishable optical inputs, combination of such distinguishable inputs is usually preferred. Such power combiners are often referred to as wavelength combiners, since radiation at different wavelengths is combined into a single spatial mode combined output. As used herein, optical power combining is understood to include both wavelength combining and/or polarization combining.
A typical wavelength combiner has two or more input ports and a single output port, where each input port i has a corresponding wavelength acceptance range Δλi which is efficiently coupled to the output port. The wavelength ranges Δλi are generally substantially non-overlapping. One way to utilize such a wavelength combiner to combine the outputs of several lasers is to provide each laser with a separate input port, such that each laser has a fixed emission wavelength within the acceptance range Δλ for the corresponding input port. While this approach is straightforward, it suffers from the disadvantage that providing lasers having emission wavelengths within the specified ranges may be costly in cases where the ranges are narrow. This cost issue is especially notable in cases where a large number of lasers are to be combined.
An alternative approach uses diode lasers and optical power combining and provides each diode laser with an input port, where each laser has an emission wavelength which may be in any of the wavelength ranges Δλi. Diode lasers tend to have a broad gain bandwidth, and the use of such lasers, for example Fabry-Perot diode lasers, is well known in the art. In this approach, a broadband partial reflector is optically coupled to the output port of the wavelength combiner. The combination of wavelength combiner and reflector provides wavelength-dependent feedback to each laser source. This linear feedback acts to set the emission wavelength of each laser source appropriately for wavelength combining. For example, a laser source coupled to a port j having an acceptance range Δλj will receive more feedback in the range Δλj than at other wavelengths, which will tend to force this source to lase at a wavelength within the range Δλj. With this approach, multiple diode lasers may be wavelength combined without the need for precise wavelength control of each laser diode.
However, certain problems which may arise in the context of wavelength combining are not addressed by the above approaches. When either of the-above approaches is used, for example, in the context of wavelength combining to provide pump radiation for a parametric nonlinear optical process which is efficient over a relatively broad wavelength range, the resulting pump radiation has a pump spectrum that is independent of the nonlinear optical process conversion efficiency. Since the pump spectrum remains fixed, careful and costly design of the broadband optical frequency converter may be required to obtain roughly constant conversion efficiency within the desired wavelength range.
There is, therefore, a need for an illumination module based on optical power combining for providing pump radiation to an optical frequency converter that automatically equalizes conversion efficiency within a conversion wavelength range.